This article presents and argues for modal structuralism, which is loosely derived from a position described by Wilfrid Sellars. Modal structuralism holds that a fundamental property is identified by the role it plays in the structure of possibilities. It implies necessitarianism about laws, which holds that at least some laws of nature are metaphysically necessary. The argument for these positions derives from the following assumptions: the principle of the identity of indiscernible properties and a modest antiquidditism. These assumptions are weaker than those of causal structuralism, which is a closely related view.